1. Field of the Invention
This invention relates to positive temperature coefficient (PTC) circuit protection devices.
2. Introduction to the Invention
PTC circuit protection devices are well known. Under normal operating conditions of the circuit, a PTC circuit protection device is in a low temperature, low resistance state. However, if a fault occurs (eg. if the current through the PTC device increases excessively, and/or the ambient temperature around the device increases excessively, and/or the normal operating current is maintained for more than the normal operating time), then the PTC device will be "tripped", i.e. converted into a high temperature, high resistance state such that the current in the circuit is reduced to a safe level. Generally, the PTC device will remain in the tripped state, even if the fault is removed, until the device has been disconnected from the power source and allowed to cool. In a batch of PTC devices made by the same manufacturing process, uncontrollable variations in the process can cause substantial variation in the conditions which will trip any individual device. The largest steady state current which will not cause any of the devices in the batch to trip is referred to herein as the "pass current" (I.sub.PASS) or "hold current", and the smallest steady state current which will cause all of the devices to trip is referred to as the "trip current" (I.sub.TRIP). In general, the difference between I.sub.PASS and I.sub.TRIP decreases slowly as the ambient temperature increases. Depending on the particular type of device, I.sub.TRIP may for example be 1.5 to 2.5 times I.sub.PASS at 20.degree. C. For any individual device, the pass current and the trip current are the same. However, in this specification, reference is made to a PTC device having an I.sub.PASS and a different I.sub.TRIP, because as a practical matter, the manufacturer of a wiring system must make use of PTC devices taken from a batch of such devices. Generally, the higher the ambient temperature, the lower the pass current and the trip current. This phenomenon is referred to as "thermal derating", and the term "derating curve" is used to denote a graph of temperature against pass current.
Generally, the way in which pass current changes with ambient temperature i.e. the shape of the derating curve, depends mainly on the PTC material, while the pass current at any particular temperature depends also on the other factors which determine the resistance and the thermal transfer characteristics of the device. Thus, using a given PTC material, it is possible to make devices which have different pass currents but derating curves of the same shape. But to make a PTC device having a derating curve of a different shape, a different PTC material must be employed.
PTC circuit protection devices contain PTC elements which may be composed of a PTC conductive polymer or a PTC ceramic, eg. a doped barium titanate. PTC ceramics have been used commercially for several decades, but they are brittle and their resistivities are higher than is desirable. Over the last decade, PTC conductive polymers which do not suffer from these disadvantages have been developed, and the use of circuit protection devices based on them has increased rapidly. Reference may be made for example to U.S. Pat. Nos. 4,237,441, 4,255,698, 4,238,812, 4,315,237, 4,426,633, 4,780,598, 4,800,253, 4,907,340, and 5,089,801, and copending commonly assigned U.S. application Ser. Nos. 07/893,626, now abandoned, 07/894,119 now U.S. Pat. No. 5,378,407, and 07/910,950 now abandoned. The disclosure of each of those patents and applications is incorporated herein by reference for all purposes.
In automobiles and other road vehicles, there is always a risk that in one way or another, e.g. through an insulation failure, one or more of the insulated wires or other components of the electrical wiring system, e.g. connectors. splices, connection blocks, switches and operative devices, will be accidentally grounded (i.e. connected, e.g. via the chassis or bodywork of the vehicle, to the ground pole of the battery), thus creating a short circuit current which, if maintained for too long, can overheat the wire (or another component in the short circuit), and thus cause damage, e.g. as a result of melting the insulation on the wire. It is conventional, in order to prevent such damage, to protect each of the wires by a fuse which, if the wire is accidentally grounded, will blow before the wire can overheat. The fuses are generally placed in fuse boxes which are readily accessible, so that blown fuses can be easily replaced. The system may also contain one or more circuit breakers for substantially the same reason; however, circuit breakers generally cost more than fuses and tend to lose their calibration (i.e. the current required to trip them changes with time and usage).
The number of operative electrical devices in automobiles and other road vehicles has risen sharply over recent years, and is still increasing. For example, many automobiles now contain electric motors to control windows, seats and door locks, and a wide range of sensors and other diagnostic apparatus, as well as interior and exterior lights. With the large number of electrical devices now present in automobiles., it is impractical to have a separate fuse to protect the wiring for each device. Many of the fuses, therefore, must protect the wiring for a number of different devices. Such shared fuses must have a current-carrying capacity substantially greater than the sum of the maximum current requirement of each of those devices, taking into account also any transient inrush currents which any electric motors, lamps or other devices may draw when first switched on. The relatively cheap fuses conventionally used in automobiles have very rapid response times, and when using such fuses, transient inrush currents often determine the capacity of the fuse, which may need to be larger than that required to handle steady state currents. Consequently, it is necessary to use wires having a still greater current-carrying capacity, even in branches which serve only devices having much smaller current requirements, and even though the fuse capacity may be determined by inrush currents which would not damage the wire.
Even with such shared fuses, however, and even with two or more fuse boxes at different locations in the vehicle, it is a significant design problem to find appropriate, accessible location(s) for the fuse box(es) without taking up space which is needed for other purposes. In addition, the use of shared fuses results in disconnection of all the devices protected by that fuse, even if only the wiring to one of them is shorted out. Present wiring systems are, therefore, expensive; they are difficult to install and maintain; they add significantly to the weight of the vehicle and, therefore, have an adverse effect on fuel consumption; and they result in disconnection of groups of devices, whereas separate wiring for each of the devices would result in disconnection of only one of the devices.
It has been proposed to replace fuses in a conventional automobile wiring harnesses by PTC devices, in order to take advantage of the fact that a PTC device can be reset without physical access to it, thus making it possible to place the PTC devices in locations which are inaccessible (and which would not, therefore be satisfactory for conventional fuses); see published German Patent Application No. P 40 15 816.